What are the criteria for first degree Atrioventricular (AV) block?

From the Guidelines

First degree atrioventricular block is characterized by a prolonged PR interval greater than 200 milliseconds on an electrocardiogram, with all atrial impulses still being conducted to the ventricles. This condition is often referred to as first-degree atrioventricular delay, as true block is not present 1. The key diagnostic criterion is the consistent PR interval prolongation across all cardiac cycles, with normal progression of the impulse from the atria to the ventricles.

Key Characteristics

• Prolonged PR interval > 200 ms
• All atrial impulses are conducted to the ventricles
• No dropped beats
• Can be physiologic (e.g., in athletes due to high vagal tone) or pathologic (e.g., due to medications, ischemic heart disease, or degenerative conduction system disease) 1

Diagnostic Considerations

• Careful evaluation of the ECG is required to diagnose atrioventricular block
• A 1:1 relationship between P waves and QRS complexes may not be present in certain scenarios, such as isorhythmic dissociation or sinus bradycardia coupled with an accelerated junctional rhythm 1
• Atrial bigeminy can lead to an erroneous diagnosis of atrioventricular block if not carefully evaluated

Clinical Implications

• First degree AVB is often asymptomatic and frequently discovered incidentally during routine ECG evaluation
• While usually benign, first degree AVB may warrant monitoring as it can sometimes progress to higher-degree blocks, particularly when the PR interval exceeds 300 milliseconds 1

From the Research

Criteria for First Degree Atrioventricular (AV) Block

The criteria for first degree AV block include:

• A prolongation of the PR interval beyond the upper limit of what is considered normal, generally > 0.20 s 2
• A PR interval of ≥ 0.30 s is considered a marked first-degree AV block, which can cause symptoms due to inadequate timing of atrial and ventricular contractions 2, 3
• A PR interval > 200 ms can also be used to diagnose first-degree AV block, especially in patients with atrial fibrillation and atrial flutter 4

Key Characteristics

• First-degree AV block is defined as a delay within the AV conduction system, without actual block 2
• The condition can be associated with atrioventricular dissociation and pseudo-pacemaker syndrome, especially in cases with extremely prolonged PR intervals 3
• The prevalence of first-degree AV block is higher in patients with atrial flutter and atrial fibrillation, and can be due to intra-atrial conduction delay 4

Diagnostic Considerations

• Electrocardiogram (ECG) is used to diagnose first-degree AV block, with a focus on the PR interval and P-wave duration 4
• A treadmill stress test may be necessary to evaluate symptoms and PR interval adaptation during exercise 5
• Patients with marked first-degree AV block may require pacemaker management, with consideration of biventricular devices in cases with left ventricular systolic dysfunction and heart failure 5